Load Regulation, Line Regulation and Cross-LoadingDC Output Load Regulation
Of course one of the first things we want to see is how well this PSU can regulate the DC outputs and maintain stable voltages. To simulate real world and maximum loading conditions, the Zalman ZM750-HP PSU was connected to the load testers and supplied with a constant 115 VAC. In this test we are interested in seeing how well a PSU can maintain the various output voltages while operating under different loads.
The new ATX12V V2.2 tolerance for voltages states how much each output (rail) is allowed to fluctuate and has tighter tolerances now for the +12V outputs. I have also included a second table of expanded tolerances (±1% to ±6%) for reference.
The following tables list the DC voltage results at the different loads for the ZM750-HP PSU while operating on 115 VAC, 60 Hz.
The PSU produced very good load regulation on all of the outputs across a broad range of loads; even when delivering the maximum rated capacity of 750 watts DC power. All of the primary rails were particularly good, staying well within 2% to 3% of the nominal voltage instead of the recommended +/-5%.
DC Output Line Regulation
In this test we are interested in seeing how well a PSU can maintain the various output voltages while the AC input line voltage changes. In the previous Load Regulation test, the AC line voltage was held constant at 115 VAC. Now we will look at how much the DC outputs change as the load is held constant and the AC line voltage is changed from 120 VAC down to 90 VAC.
The Line Regulation test was performed with the combined DC loads set to 600W. The AC input voltage to the power supply (via the Extech power analyzer) was adjusted using a Powerstat variable autotransformer.
Virtually no measurable change in the DC outputs; very good.
PC switching mode power supplies provide multiple DC output voltages. Ideally, the total load should be distributed across all the main outputs (+3.3V, +5V, +12V). This means that the combined +3.3V and +5V load should be proportional to the combined +12V load – as one increases, so should the other. Unfortunately, this is not always the case, especially in newer PCs that predominately use +12V and may put only minimal loads on the +3.3V and/or +5V rails.
Cross-loading refers to imbalanced loads. If a PC pulls 400W on the +12V outputs and only 40W (or less) on the combined 3.3V and +5V outputs, the resulting voltage regulation may suffer.
In the first test we put a heavy load (60A) on the +12V output and a light load on the remaining outputs. The ZM750-HP PSU had no problem delivering the maximum rated load of 720W on the combined +12V rails. Even with this large imbalance, the voltages all stayed well within spec.
In the second test we reversed the cross-load and placed a heavy load (146W) on the +3.3V and +5V outputs with a light load (2A/24W) on the +12V rail. Once again, the PSU passed this test without problems with all the voltages looking very good.